Technical Field
The present disclosure relates to external compensation for a display device and a method of driving the same.
Discussion of the Related Art
Various types of panel displays have been developed and sold. Among the various types of panel displays, an electroluminescent display is classified into an inorganic electroluminescent display and an organic electroluminescent display depending on a material of an emission layer. In particular, an active matrix organic light emitting diode (OLED) display includes a plurality of OLEDs capable of emitting light by themselves and has many advantages, such as fast response time, high emission efficiency, high luminance, wide viewing angle, and the like.
An OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When power (voltage) is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML and form excitons. As a result, the emission layer EML generates visible light.
An OLED display includes a plurality of pixels, each including an OLED and a thin film transistor (TFT) that adjusts a luminance of an image implemented on the pixels based on a grayscale of image data. The driving TFT controls a driving current flowing into the OLED depending on a voltage (hereinafter, referred to as “a gate-to-source voltage”) between a gate electrode and a source electrode of the driving TFT. An amount of light emitted by the OLED is determined depending on the driving current of the OLED, and the luminance of the image is determined depending on the amount of light emitted by the OLED.
In general, when a driving TFT operates in a saturation region, a pixel current Ids flowing between a drain electrode and a source electrode of the driving TFT is expressed by the following Equation 1.Ids=½*(μ*C*W/L)*(Vgs−Vth)2  [Equation 1]
In the above Equation 1, μ is electron mobility, C is a capacitance of a gate insulating layer, W is a channel width of the driving TFT, and L is a channel length of the driving TFT. In addition, Vgs is a voltage between a gate electrode and a source electrode of the driving TFT, and Vth is a threshold voltage (or a critical voltage) of the driving TFT. A gate-to-source voltage Vgs of the driving TFT may be a voltage differential between a data voltage and a reference voltage in accordance with a pixel structure. The data voltage is an analog voltage corresponding to a grayscale of image data, and the reference voltage is a fixed voltage. Therefore, the gate-to-source voltage Vgs of the driving TFT is programmed or set depending on the data voltage. Then, the pixel current Ids is determined depending on the programmed gate-to-source voltage Vgs.
Electrical characteristics of the pixel, such as the threshold voltage Vth and the electron mobility pt of the driving TFT and a threshold voltage of the OLED, may be factors determining an amount of pixel current Ids of the driving TFT. Therefore, all the pixels should have the same electrical characteristics. However, a variation in the electrical characteristics between the pixels may be caused by various factors such as manufacturing process characteristics and time-varying characteristics. The variation in the electrical characteristics between the pixels may lead to a luminance variation, and it is difficult to implement desired images or meet image quality requirements.
In order to compensate for the luminance variation between the pixels, there are so-called external compensation techniques for sensing electrical characteristics of the pixels and correcting (or compensating for) an input image based on the sensing result. In order to compensate for the luminance variation, a current change by an amount of Δy has to be ensured when the data voltage applied to the pixel is changed by an amount of “Δx”. Thus, the external compensation technique is to implement the same (or effectively the same) brightness by calculating “Δx” for each pixel and applying the same pixel current to the OLED. Namely, the external compensation technique may be implemented to adjust the gray levels so that the pixels have the same or effectively the same brightness.
The electrical characteristics of the pixels may continuously change during the driving of the pixels. Thus, a real-time compensation technique for compensating for changes in the electrical characteristics of each pixel in real time may be needed to increase an external compensation performance.
In order to implement such real-time compensation techniques, a method has been proposed to perform a sensing drive operation in a vertical blanking interval, in which input image data is not written. The vertical blanking interval is disposed between every adjacent vertical active period in which input image data is written in one frame. A related art driving circuit for external compensation senses one display line in a vertical blanking interval of each frame period. To this end, a gate driver included in the related art driving circuit for external compensation generates a sensing gate signal during the vertical blanking interval and applies the sensing gate signal to pixels formed on a sensing target display line. The gate driver includes a plurality of cascade-connected stages.
A length of the vertical blanking interval is much shorter than a length of the vertical active period. Because each of the stages constituting the gate driver receives an output signal of a previous stage as a carry signal and sequentially operates in response to the carry signal, limited time of the vertical blanking interval may be insufficient to generate a desired sensing gate signal. For example, an Nth sensing gate signal generated in an Nth stage is necessary to sense an Nth display line of a display panel having a vertical resolution of “N”. However, because the Nth stage is driven after all of the first to (N−1)th stages are sequentially driven, all of the N stages included in the gate driver have to be driven to generate the Nth sensing gate signal. However, one vertical blanking interval does not provide enough time to operate all the stages of the gate driver. Such a problem is magnified and becomes more significant as the vertical resolution of the display panel increases and as the number of display lines to be sensed in one vertical blanking interval increases.